Table 5 Grading for strength of evidence [18]

Strong

≥ 2 high-quality studies showing positive association between the presence of a variable and the outcome

AND

No studies showing a negative association

Moderate

One high-quality AND one lesser-quality study showing association

AND

No studies showing negative association

Weak

> 2 low-quality studies showing positive association

OR

Only one high-quality study showing positive association

Negative

≥ 1 high-quality study showing negative association (inverse relationship)

AND

No studies showing a positive association

Inconclusive

Associations present in only low-quality study

OR

No studies of any quality showing univariate association

OR

Presence of positive and negative associations from different articles, regardless of study quality